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United States Patent |
5,705,449
|
Hirao
,   et al.
|
January 6, 1998
|
High-strength, high-toughness silicon nitride sinter
Abstract
A method for the production of a high-strength high-toughness silicon
nitride sinter includes the steps of mixing a silicon nitride powder with
a sintering additive, adding to the resultant mixture as seed particles
0.1 to 10% by volume, based on the amount of the mixture, of elongated
single crystal .beta.-silicon nitride particles having a larger minor
diameter than the average particle diameter of the silicon nitride powder
and having an aspect ratio of at least 2, forming the resultant mixture so
as to orient the elongated single crystal .beta.-silicon nitride particles
as seed particles in a specific direction, and heating the green body to
density it and simultaneously induce epitaxial growth of single crystal
.beta.-silicon nitride particles, and a high-strength, high-toughness
silicon nitride sinter obtained by the method.
Inventors:
|
Hirao; Kiyoshi (Nagoya, JP);
Brito; Manuel E. (Nagoya, JP);
Kanzaki; Shuzo (Kasugai, JP)
|
Assignee:
|
Agency of Industrial Science & Technology, Ministry of International (Tokyo, JP)
|
Appl. No.:
|
761206 |
Filed:
|
December 6, 1996 |
Foreign Application Priority Data
| Sep 20, 1994[JP] | 6-253095 |
| Dec 21, 1994[JP] | 6-336177 |
Current U.S. Class: |
501/97.1; 501/97.2 |
Intern'l Class: |
C04B 035/587 |
Field of Search: |
501/97,97.1,97.2
|
References Cited
U.S. Patent Documents
4994219 | Feb., 1991 | Yeh | 501/97.
|
5171723 | Dec., 1992 | Moriguchi et al. | 501/97.
|
5401450 | Mar., 1995 | Mitomo et al. | 501/97.
|
5545597 | Aug., 1996 | Yeckley | 501/97.
|
Primary Examiner: Gruop; Karl
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Parent Case Text
This is a division of application Ser. No. 08/529,854 filed on Sep. 18,
1995, pending.
Claims
What is claimed is:
1. A high-strength, high-toughness silicon nitride sinter prepared by:
mixing a silicon nitride powder and a sintering additive;
mixing 0.1 to 10% by volume, based on the amount of silicon
nitride-sintering additive mixture, of elongated single crystal
.beta.-silicon nitride seed particles having a larger minor diameter than
the average particle diameter of said silicon nitride powder and having an
aspect ratio of at least 2, into said silicon nitride-sintering additive
mixture;
shaping the resultant mixture by a means which orients said elongated
single crystal .beta.-silicon nitride seed particles, in their elongated
dimension, coplanar with a plane defined by the direction of movement of
said means, thereby preparing a green body; and
sintering said green body, thereby simultaneously densifying said green
body and inducing epitaxial growth of said single crystal .beta.-silicon
nitride seed particle thereby obtaining a silicon nitride sinter having a
strength of not less than 1100 MPa and a fracture toughness of not less
than 11 MPa m.sup.1/2.
2. The sinter according to claim 1, wherein the silicon nitride
powder-sintering additive-seed crystal mixture is mixed with an organic
binder.
3. The sinter according to claim 2, wherein, prior to sintering, the green
body is calcined at 600.degree. C. to 1000.degree. C. in order to remove
organic binder from the green body.
4. The sinter according to claim 1, wherein said shaping means is a doctor
blade.
5. The sinter according to claim 1, wherein said shaped green body is in
the form of a green sheet.
6. The sinter according to claim 1, wherein the large elongated grains
within said sintered body resulting by the induced epitaxial growth of
said single crystal .beta.-silicon nitride seed particles accounts for a
volume amount of not less than 10%.
7. The sinter according to claim 1, wherein said heated green body is
sintered at a temperature ranging from 1,700.degree. to 2,000.degree. C.
under an inert gas pressure of 1 to 200 atmospheres.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a silicon nitride sinter exhibiting extremely
high strength and toughness in a specific direction and a method for the
production thereof.
Silicon nitride exhibits higher covalent bond strength and much better
stability at high temperatures than oxide ceramics. This has stimulated
research into high-temperature structural materials using silicon nitride.
While some practical applications have recently been found for silicon
nitride such as in engine parts, including automobile grade turbo
chargers, use has been limited because, despite being the toughest among
ceramics, silicon nitride has a fracture toughness that is one or more
decimal places lower than that of metallic materials.
2. Description of the Prior Art
Various methods aimed at further improving the toughness of silicon nitride
ceramics have been studied. Among these, the method of improving toughness
by dispersing plate- or column-shaped second phase in the ceramic matrix
and consequently producing a bridging or an pull-out effect along cracks
occurring in the ceramic matrix proves particularly effective. Various
procedures have been developed for implementing this method. These include
the method of dispersing whiskers or platelike particles by mechanical
agitation and the method of developing coarse columnar grains of
.beta.-silicon nitride in the sinter as by gas pressure sintering.
The former method produces a highly toughened silicon nitride sinter having
fracture toughness in the range of from 10 to 14 MPa.multidot.m.sup.1/2.
Am Ceram. Soc. Bull., 65›2! 351-356 (1986), for example, reports formation
of a sinter having fracture toughness in the range of from 10 to 12
MPa.multidot.m.sup.1/2 by dispersing 10 to 40% in volume of SiC whiskers
in a silicon nitride and subjecting the resultant green body to hot press
sintering, and Ceramic Transactions, Vol. 19, pp. 765-771 reports
production of a sinter having fracture toughness of 14
MPa.multidot.m.sup.1/2 by dispersing 30% in volume of SiC platelike
particles in a silicon nitride and subjecting the resultant green body to
a treatment with a hot press. Though the sinters obtained by these
procedures have high levels of fracture toughness, they exhibit
conspicuously low strength (in the range of from 400 to 600 MPa) because
the incorporated reinforcing materials act as flaws. Besides, the
dispersion of 10 to 40% in volume of a second phase is expensive because
it requires firing by a special method such as hot pressing or hot
isostatic pressing (HIP).
The latter method consists in firing in an ambience of nitrogen compressed
to about 100 atmospheres at a temperature in the range of from
1800.degree. to 2000.degree. C. thereby developing large elongated
.beta.-silicon nitride grains in the sinter that produce the same effect
as whiskers. This method eliminates the need for hot press or HIP and
forms a silicon nitride sinter having high fracture toughness in the range
of from 8 to 11 MPa.multidot.m.sup.1/2. This is comparable to the
toughness obtained by incorporation of whiskers.
Am. Ceram. Soc. Bull., 65›9! 1311-1315 (1986) reports production of a
silicon nitride sinter having fracture toughness of about 9
MPa.multidot.m.sup.1/2 by adding alumina-rare earth oxide as a sintering
additives to raw material .alpha.-Si.sub.3 N.sub.4 and firing the
resultant mixture in an ambience of nitrogen compressed to 20 to 40
atmospheres at 2000.degree. C., and J. Am. Ceram. Soc., 76›7! 1892-1894
(1993) reports production of a silicon nitride sinter having fracture
toughness in the range of from 8.5 to 10.3 MPa.multidot.m.sup.1/2 by
adding Y.sub.2 O.sub.3 -Nd.sub.2 O.sub.3 as a sintering additives to raw
material of .beta.-Si.sub.3 N.sub.4 and firing the resultant mixture in an
ambience of nitrogen compressed to 100 atmospheres at 2000.degree. C. for
2 to 8 hours. The high-toughness silicon nitride sinters obtained by these
gas pressure sintering methods have low strength (in the range of from 400
to 700 MPa) because large elongated .beta.-silicon nitride grains
developing in the sinter act as flaws.
No silicon nitride sinter which combines high strength with high toughness
has yet been developed. It is, therefore, a primary object of this
invention to provide a silicon nitride sinter combining high strength with
high toughness and enabling production thereof by an inexpensive process,
and a method for the production thereof.
SUMMARY OF THE INVENTION
Specifically, this invention relates to a method for the production of a
high-strength, high-toughness silicon nitride sinter comprising the steps
of mixing a silicon nitride powder with a sintering additive, adding to
the resultant mixture as seed particles 0.1 to 10% by volume, based on the
amount of the mixture, of rod-like single crystal .beta.-silicon nitride
particles having a larger diameter than the average particle diameter of
the silicon nitride powder and having an aspect ratio of at least 2,
forming the resultant mixture so as to orient the rod-like single crystal
.beta.-silicon nitride particles, added as seed particles in a specific
direction, and the green body in heated to density it and induce epitaxial
growth from single crystal .beta.-silicon nitride particles; resulting in
the high-strength, high-toughness silicon nitride sinter obtained by this
method.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a model diagram showing the microstructure of a silicon nitride
sinter obtained by this invention.
FIG. 2 is an electron micrograph at 2000 magnifications of a fracture
surface of the sinter of this invention.
FIG. 3 is an electron micrograph at 2000 magnifications of a ground and
etched surface of the sinter of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
First, the research that led to this invention will be summarized.
Prior to achieving this invention, the inventors extensively studied the
conventional high-toughness silicon nitride sinter obtained by gas
pressure sintering for determining the relation between the
microstructure, strength, and fracture toughness of the sinter. They found
that the conventional gas pressure sintering causes random growth of
elongate grains from .beta.-silicon nitride particles present in the raw
material silicon nitride as nuclei and consequently induces occurrence of
large elongate grains in the produced sinter. Since the large elongate
grains are origin for fracture, the sinter presents low strength, although
it has improved fracture toughness.
In cooperation with their colleagues, the inventors prepared seed crystals
and produced a sinter by repeating the method disclosed in J. Am. Ceram.
Soc., 77›7! 1857-1862 (1994). To be specific, they added rod-like single
crystal .beta.-silicon nitride particles morphological regulated as seed
crystals to silicon nitride raw material and tried to control the shape
and size of anisotropically grown elongate .beta.-silicon nitride grains.
They consequently succeeded in producing a silicon nitride sinter having
relatively high strength in the range of from 900 to 1000 MPa and
relatively high toughness in the range of from 8.2 to 8.6
MPa.multidot.m.sup.1/2. When the seed crystals were enlarged or the amount
of seed crystals added was increased for the purpose of further enhancing
the toughness, however, the strength declined to about 700 MPa in spite of
improvement of the fracture toughness to a level in the range of from 9 to
10 Mpa.multidot.m.sup.1/2.
The inventors next investigated the microstructure of the sinter with
respect to the relation between the strength and toughness thereof and
confirmed the following fact.
They made a thorough observation of the microstructure of the system under
discussion to find that, in a system retaining high strength and acquiring
enhanced toughness, a group of large elongated grains developed from seed
crystals were dispersed in a matrix of small grains and, in a system
suffering a decline in strength, the group of large elongated grains grown
from the seed crystals coalesced. This coalescence was suspected of
causing an increase in the flaw size.
Based on this knowledge the inventors continued their study on control of
the microstructure by the incorporation of seed crystals. They
consequently learned that when the group of large elongated grains present
in random three-dimensional orientation are aligned in a specific
direction, the toughness of the system is effectively improved in that
direction without decrease in strength. Ultimately, they obtained the
high-strength, high-toughness silicon nitride sinter of this invention.
This invention will now be described in detail.
To manufacture the high-strength, high-toughness silicon nitride sinter,
this invention requires the raw material powder of silicon nitride to be
added with a prescribed amount of a sintering additive. The silicon
nitride raw material may be in any of such crystal systems as .alpha.
type, .beta. type, or a morphous type. It is advantageously used in the
form of a fine powder having an average particle diameter of not more than
0.5 .mu.m. The sintering additive may be any of the known compounds
available for the purpose of accelerating the sintering. Concrete examples
are MgO, CaO, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, Yb.sub.2 O.sub.3,
HfO.sub.2, Sc.sub.2 O.sub.3, CeO.sub.2, ZrO2, SiO.sub.2, Cr.sub.2 O.sub.3,
and AlN.
The combination of these sintering additives and the amount of sintering
additive to be added vary with the method of firing, which may, for
example, be normal pressure sintering, gas pressure sintering, hot press,
or hot isostatic pressing (HIP). They are so selected that the sample, on
being fired by a given method, is compacted to a relative density of not
less than 97%. For enabling the silicon nitride to attain anisotropic
growth in an elongated shape during sintering, the sintering additive
should appropriately contain a rare earth oxide such as Y.sub.2 O.sub.3 or
Yb.sub.2 O.sub.3.
The mixing of the raw materials can be conducted using any of the
commercially available equipments available for mixing or blending
powders. The raw materials are advantageously mixed wet by the use of a
suitable solvent such as water, methanol, ethanol, or toluene. In the wet
mixing, it is best to use an organic solvent for preventing the otherwise
possible oxidation of silicon nitride. In the presence of such an organic
solvent, the mixing can be effectively accelerated by using a dispersant
such as sorbitan monooleate.
Then, to the slurry obtained as described above, rod-like single crystal
.beta.-silicon nitride particles are added as seed crystals in an amount
in the range of from 0.1 to 10% by volume, preferably from 1 to 5% by
volume. If the amount of the seed crystals so added is less than 0.1% by
volume, the group of elongated grains will not be incorporated in a fully
satisfactory amount into the sinter. Conversely, if the amount exceeds 10%
by volume, the excess of added seed crystals impede the sintering to the
extent of preventing the formation of a compact sinter and, though a
compact sinter may be attained by pressure sintering such as hot press,
the excessive seed crystals will prevent the produced sinter from
acquiring high strength because the group of elongated grains grown from
the seed particles coalesce and increase the size of flaw. Hence, the
amount of the seed crystals added should be limited to the range of from
0.1 to 10% by volume. The shape of the seed crystals is preferably such
that the diameter is larger than the average particle diameter of the raw
material powder of silicon nitride and the aspect ratio is not less than
2. If the diameter of the seed crystals is smaller than the average
particle diameter of the raw material powder, the seed crystals will be
dissolved in the transient liquid during sintering and will not accomplish
their role of seed crystals. If the aspect ratio is not more than 2, the
seed crystals will not be thoroughly oriented as in the case of sheet
molding and will induce coalescence between the randomly grown elongated
grains and consequently cause the produced sinter to suffer a decrease in
strength. The upper limit of the aspect ratio is about 50. If the aspect
ratio exceeds 50, the seed crystals will not be thoroughly dispersed.
The elongated single crystal .beta.-silicon nitride particles used as seed
crystals may be commercially available .beta.-silicon nitride whiskers.
However, since these whiskers lack uniformity of size and contain lattice
defects and impurities, it is better to use rod-like single crystal
.beta.-silicon nitride particles of high purity and uniform size
manufactured by a method such as that reported in Journal of Ceramic
Society of Japan, 101›9! 1071-1073 (1993). It is important that the
addition of the seed crystals to the raw material powder be implemented by
mixing the silicon nitride raw material thoroughly with the sintering
additive in accordance with the wet mixing technique mentioned above and
causing the seed crystals to be dispersed in the resultant slurry by means
of ultrasonic dispersion or by the pot mixing technique using a resin pot
and coated resin balls in such a manner as to avoid breaking the seed
crystals.
Then, the mixed slurry obtained as described above and a proper amount of
an organic binder added thereto are mixed. The produced mixture is sheet
molded by the use of a doctor blade or formed by the use of an extrusion
device to effect orientation of the seed crystals in the mixture.
Particularly when the mixture is sheet molded, the produced sheet are
stacked using a hot plate press to acquire a prescribed thickness.
Subsequently, the formed mixture is calcined by the ordinary firing
schedule, i.e. at a temperature in the approximate range of from
600.degree. to 1000.degree. C. to remove the binder and then fired in the
ambience of nitrogen kept at a temperature in the range of from
1700.degree. to 2000.degree. C. under a pressure of 1 to 200 atmospheres.
For the purpose of obtaining a sinter to manifest high strength and high
toughness, it is important to sinter to a relative density of not less
than 97% and that the elongated .beta.-silicon nitride grains be
thoroughly developed from the seed crystals. The silicon nitride sinter
obtained from the raw material which incorporates the seed crystals
possesses a microstructure in which large elongated .beta.-silicon nitride
grains epitaxially grown from the seed crystals are two-dimensionally
dispersed in a matrix of small silicon nitride grains. It is important
that the group of these large elongated grains account for a volume ratio
of not less than 10%. If the volume ratio of the group of elongated grains
after the firing is less than 10%, the level of improvement of toughness
will be unduly low and the desired sinter will not be obtained. The
specific temperature, nitrogen pressure, and keeping time during the
firing are closely related to the sintering additive. It is, therefore,
necessary to decide the optimum conditions for enabling a given sintering
additive to fulfill the requirements mentioned above and for enabling the
produced sinter to manifest high strength and high toughness by
preliminary testing, for example. This high-toughness silicon nitride
sinter is characterized by being compacted by pressureless firing or gas
pressure firing. Optionally, the densification may be effected by hot
press or HIP.
The silicon nitride sinter produced by the method of this invention as
described above possesses such a microstructure that elongated
.beta.-silicon nitride grains grown epitaxially from .beta.-silicon
nitride particles as seeds are highly dispersed in a planar distribution.
Owing to the microstructure of the sinter which has the elongated grains
oriented in a planar distribution as described above, this sinter acquires
enhanced strength in a direction perpendicular to the direction of the
orientation (1) because the whole group of elongated grains function
toward enhancing the toughness and consequently the level of improvement
of toughness is high as compared with the conventional high-toughness
silicon nitride in which the group of elongated grains are present in
random three-dimensional orientation and (2) because the large elongated
grains, though present in the texture of the sinter, are dispersed in a
planar distribution and therefore the extent to which they act as flaw is
small.
This invention allows production of a silicon nitride sinter which acquires
compaction to a relative density of not less than 99% and exhibiting a
strength of not less than 1100 MPa and a fracture toughness of not less
than 11 MPa.multidot.m.sup.1/2 in a direction perpendicular to the
direction of orientation of the elongated grains.
The microstructure of the silicon nitride sinter obtained by this invention
is shown by a model diagram in FIG. 1.
In FIG. 1, the symbol "a" represents a small silicon nitride grains, the
symbol "b" an rod-like single crystal .beta.-Ni.sub.3 N.sub.4 particles as
a seed crystal, and the symbol "c" elongated grains grown epitaxially from
a seed crystal.
The method disclosed by this invention enables production of a silicon
nitride sinter exhibiting strength of not less than 1100 MPa and fracture
toughness of not less than 11 MPa.multidot.m.sup.1/2 in a direction
perpendicular to the direction of orientation of the elongated grains and
consequently permits provision of a silicon nitride ceramic simultaneously
presenting strength and toughness of a high level unattainable by
conventional silicon nitride ceramics.
The silicon nitride sinter of this invention therefore possesses
outstanding reliability as compared with conventional silicon nitride
sinters and can be expected to find extensive utility as a structural
material for heat exchangers, engines, and gas turbine parts in the place
of refractory alloys.
The invention will now be described below with reference to working
examples and comparative examples.
EXAMPLE
Production of seed Crystals
In a planetary mill using balls and a pot both made of silicon nitride, 30
g of a raw material powder of .alpha.-Si.sub.3 N.sub.4 having a specific
surface area of 2 m.sup.2/g and 2.418 g of Y.sub.2 O.sub.3 and 1.288 g of
SiO.sub.2 added thereto were mixed in methanol as a mixing medium
(Composition A). Similarly, 30 g of a raw material of .alpha.-Si.sub.3
N.sub.4 having a specific surface area of 5 m.sup.2 /g and 2.418 g of
Y.sub.2 O.sub.3 and 0.322 g of SiO.sub.2 added thereto were mixed
(Composition B). The compositions A and B were each treated with a vacuum
evaporator to vaporize methanol, further vacuum dried at 120.degree. C.,
and passed through a 60-mesh sieve to obtain a composite for the
preparation of seed crystals. The composite was placed in a crucible made
of silicon nitride and heated therein in an ambience of nitrogen under 5
atmospheres at 1850.degree. C. for two hours. The aggregate consequently
obtained was crushed into a powder of 60 mesh.
The powder obtained as described above was sequentially treated with an
aqueous solution of hydrofluoric acid-nitric acid mixture (hydrofluoric
acid: nitric acid: water=45:5:50 in volume percentage), sulfuric acid,
dilute hydrofluoric acid, and aqua ammonia in the order mentioned to
remove Y.sub.2 O.sub.3 and SiO.sub.2, glass phase components, and obtain
rod-like single crystal .beta.-silicon nitride particles. From Composition
A, rod-like single crystal .beta.-silicon nitride particles having a
diameter of 1.4 .mu.m and an aspect ratio of 4 (Seed Crystals SA) were
obtained. From Composition B, rod-like single crystal .beta.-silicon
nitride particles having a diameter of 0.9 .mu.m and an aspect ratio of 10
(Seed Crystals SA) were obtained. These two lots of seed crystals both
possessed extremely high purity as evinced by the fact that the oxygen
content was not more than 0.26% and the yttrium content was not more than
1.3 ppm.
Examples 1-5
Production of sinter of this invention
In a planetary mill using balls and a pot both made of silicon nitride, a
raw material powder of .alpha.-Si.sub.3 N.sub.4 (having a specific surface
area of 10 m.sup.2 /g and an average particle diameter of 0.1 .mu.m) and a
sintering additive composed of Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 and 3%
by weight of a dispersant ›produced by Kao Co., Ltd. and marketed as
"Diamine RRT"! based on the total of the other three components added
thereto were mixed for three hours in a toluene-butanol liquid mixture
(80% by volume of toluene and 20% by volume of butanol) as a mixing
medium.
The amount of the mixing medium per 100 g of solids was 110 cc. The amounts
(% by weight) of the seed crystals Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3
based on the total amount of solids (.alpha.-Si.sub.3 N.sub.4, seed
crystals Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3) in the working examples
were as shown in Table 1.
The slurry obtained in each example and the relevant seed crystals added in
an amount of 2 or 5% by weight based on the total amount of solids were
mixed for 24 hours by the use of a resin pot and resin-coated balls.
Further, the resultant mixture and 9% by weight of a binder (polyvinyl
butyral resin) and 2.25% by weight of a plasticizer (dioctyl adipate)
based on the total amount of solids which were added thereto were mixed
for 48 hours. The slurry consequently obtained was formed by the doctor
blade method into a green sheet having a thickness of 150 m.
When this green sheet was observed under an electron microscope, the seed
crystals SA and SB were both found to be oriented in a planar form in the
plane of the sheet. The green sheet was cut into rectangles of 45.times.50
mm and 50 such rectangles were superposed in one direction and laminated
at 130.degree. C. under a pressure of 70 kg/cm.sup.2. The resultant
laminate was calcined in a stream of a mixed gas of 95% N.sub.2 and 5%
H.sub.2 at 600.degree. C. for two hours to remove the organic binder. The
calcined sheet was placed in a carbon crucible, covered with a Si.sub.3
N.sub.4 powder, and retained in an ambience of nitrogen compressed to 9
atmospheres at 1850.degree. C. for six hours, to obtain a sinter of this
invention.
The sinter thus obtained was cut into two types test pieces measuring
3.times.4.times.40 mm. One type was cut so that the sheet forming
direction coincided with the longitudinal direction of the test pieces (A
direction) and the other type was cut so that the direction perpendicular
to the sheet forming direction coincided with the longitudinal direction
of test pieces (B direction). After polishing, the test pieces were tested
for specific gravity, for room temperature four-point bending strength as
specified by JIS (Japanese Industrial Standards) R-1601, and for fracture
toughness by the SEPB method specified by JIS R-1607. A sample was mirror
ground and then etched by immersion in an equimolar mixed solution of NaOH
and KOH at 280.degree. C. for 15 minutes. In the etched surface of the
sample, the ratio of surface area of the group of large elongated grains
grown from seed crystals was measured. The results of these tests are
shown in Table 1. The electron micrograph (2000 magnifications) of a
fractures surface of the sinter mentioned above is shown in FIG. 2 and the
electron micrograph (2000 magnifications) of a ground and etched plane of
the same sinter is shown in FIG. 3. In the photographs, the symbol "d"
represents the direction of sheet lamination and the symbol "e" the
direction of sheet formation. The density in Table 1 represents the
relative density (%) based on the relevant theoretical density. In FIG. 2,
the symbol "A" represents a large elongated grain (grown from a seed
crystal) and the symbol "B" a matrix of small silicon nitride grains. In
FIG. 3, the symbol "A'" represents an elongated grain and the symbol "B'"
a matrix of small silicon nitride grains. The photographs show that the
elongated grains had a prefered orientation.
The doctor blade method, JIS R-1601, and JIS R-1607 mentioned above will
now be explained.
Doctor blade method: A slurry is prepared by dissolving an organic binder
in a solvent and dispersing a given ceramic raw material in the resultant
solution. The slurry is spread thin on a carrier film by the use of a
blade. The spread layer of the slurry is dried to remove the solvent and
obtain a molded sheet having the ceramic raw material powder fixed by the
organic binder.
JIS R-1601 (Four-point bending strength measurement): A test piece is
placed on two supporting points (lower supporting points) separated by a
prescribed distance and a load is applied as split between two points
(upper load points) of the test piece separated by equal distances in
opposite directions from the center thereof between the supporting points
to find the maximum bending stress at the time the test piece fractures.
According to JIS R-1601, the distance between the lower supporting points
(outer span) is 30 mm, the distance between the upper loading point (inner
span) is 10 mm, and the total length, width, and thickness of the test
pieces are respectively not less than 36 mm, 4.0.+-.0.1 mm, and 3.0.+-.0.1
min.
JIS R-1607 (SEPB method, single-edge-precracked-beam method): The fracture
load of a test piece is determined by precracking the test piece and
subjecting this test piece to a three-point bending fracture test. The
magnitude of fracture toughness of the test piece is determined based on
the precrack length, the size of the test piece, and the distance between
the bending supporting points. According to JIS R-1607, the distance
between the supporting points is 16 or 30 mm, the width of the test piece
is 4.0.+-.0.1 mm, the thickness of the test piece is 3.0.+-.0.1 mm, and
the precrack length is 1.2 to 2.4 mm in the three-point fracture test.
COMPARATIVE EXAMPLE 1
In a planetary mill using balls and a pot both made of silicon nitride, a
raw material powder of .alpha.-Si.sub.3 N.sub.4 and a sintering additive
composed of Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 and 0.5% by weight of a
dispersant (produced by Lion Chemicals Ltd., Japan and marketed as
"Reogard GP") based on the total of the other three components added
thereto were mixed in methanol as a mixing medium for three hours. The raw
material used was the same as in Example. The composition of the
components used was as shown in Table 1. The resultant mixture was dried
with a vacuum evaporator to vaporize methanol. The dry residue was
calcined in a stream of a mixed gas of 95% N.sub.2 and 5% H.sub.2 at
600.degree. C. for two hours to remove the organic components. The
resultant mixed powder was preformed in the shape of a rectangular cube
42.times.47.times.5 mm by the use of a metal mold and further subjected to
CIP forming (cold isostatic pressing) under a pressure of 500 MPa. The
shaped solid thus manufactured was fired under the same conditions as in
Example. The produced sinter was rated in the same manner as in Example.
The results are shown in Table 1.
COMPARATIVE EXAMPLES 2-5
In a planetary mill using balls and a pot both made of silicon nitride, a
raw material powder of .alpha.-Si.sub.3 N.sub.4 and a sintering additive
composed of Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 and 0.5% by weight of a
dispersant (produced by Lion Chemicals Ltd., Japan and marketed as
"Reogard GP") based on the total of the other three components added
thereto were mixed in methanol as a mixing medium for three hours. The
resultant slurry and a prescribed amount of seed crystals added thereto
were mixed for 24 hours by the use of a resin pot and resin-coated balls.
The raw materials used herein were the same as in Example and the
composition of the components was as shown in Table 1. The resultant
mixture was dried to vaporize methanol, treated for removal of the organic
components, and then molded and fired by following the procedure of
Comparative Example 1. The sinter consequently obtained was rated in the
same manner as in Example. The results are shown in Table 1.
TABLE 1
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Properties of sinter
Seed crystals Fracture
Elongated grains
Amount added
Density
Strength
toughness
Ratio
Property of
Molding method
Sintering additive
Kind
(% by weight)
(%) (MPa)
(MPa .multidot. m.sup.1/2)
surface
orientation
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Comparative
Metal mold Press
5 wt % Y.sub.2 O.sub.3, 2 wt % Al.sub.2 O.sub.3
None 99.3
1000
6.6 None
Exper-
iment 1
Comparative
" " SA 2 99.2
890
8.7 31 None
Exper-
iment 2
Comparative
" " " 5 98.3
780
8.8 39 None
Exper-
iment 3
Comparative
" " SB 2 99.0
890
7.9 21 None
Exper-
iment 4
Comparative
" " " 5 97.5
760
8.7 30 None
Experi-
ment 5
Example 1
Sheet Sekisou
5 wt % Y.sub.2 O.sub.3, 2 wt % Al.sub.2 O.sub.3
SA 2 99.3
1150
11.5 32 Exist
*1151
*11.0
Example 2
" " " 5 99.1
1130
12.0 42 Exist
Example 3
" " SB 2 99.3
1200
11.0 23 Exist
*1150
*11.0
Example 4
" " " 5 99.2
1140
11.5 33 Exist
Example 5
" 6 wt % Y.sub.2 O.sub.3, 2 wt % Al.sub.2 O.sub.3
" " 99.3
1150
11.7 32 Exist
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Note: In the columns titled "strength" and "toughness", the unmarked
numerical values represent data obtained of samples cut in the A directio
and the asterisked (*) numerical values represent data obtained of sample
cut in the B direction.
Table 1 shows that the sinters obtained in working examples of this
invention exhibited outstanding properties as compared with the sinters
obtained in Comparative Examples as is evident from the fact that their
strengths were not less than 1100 MPa and their fracture toughnesses were
not less than 11 MPa.multidot.m.sup.1/2.
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